This invention relates to a method for producing hose, and more particularly, to a hydraulic hose embodying metallic reinforcement layers and thermoplastic core tube and cover layers. Hoses formed of thermoplastic materials have become commonplace due to the advantages in being relatively light weight and having relatively small dimensions. One of the problems encountered with such hose, however, is maintaining the integrity of the hose which requires that reliable, long lasting adhesive bonds are established between the layers of such hoses.
A further advantage of thermoplastic hoses is that thermoplastic materials such as nylons, urethanes and co-polyesters can be used which inherently possess excellent chemical resistance to a wide variety of solvents, oils, fuels and other chemicals. This advantage, however, also poses a challenge to the hose manufacturer, in that the excellent chemical resistance of these thermoplastics makes it difficult to obtain adhesives which will chemically bond to the relatively inert surfaces of the core tube and cover of the hose. In addition, the typical textile fiber reinforcements of these hoses, such as polyester, nylon or aramid yarns are also relatively inert and hard to bond. To solve these problems, hose manufacturers have found various adhesive chemicals, particularly, urethane adhesives to use for thermoplastic hoses that are textile yarn reinforced. However, until now, there has been no reliable method for adhesively bonding a thermoplastic hose where the main reinforcement is steel wire. The primary object of this invention is to produce a thermoplastic hose construction wherein the thermoplastic cover is mechanically interlocked and thereby bonded with the steel wire reinforcement which may be brass plated steel wire, stainless steel wire or any other ferrometallic material capable of being induction heated. The secondary object is to provide a controlled degree of combined mechanical and chemical bonding between the cover and the core tube material.
An example of prior art in the construction of steel wire reinforced thermoplastic hose is in U.S. Pat. No. 4,341,578 of Chermak et al, who teaches the localized heating of the braided wire layer by an induction heating process. In the Chermak patent, the induction heating of the wire braid is performed during the braiding process and causes localized generation of heat in the steel wire which is conducted to the outer surface of the thermoplastic core tube. As a result, thermal expansion of the core tube is achieved, which results in protrusion of core tube material through interstices and small openings in the wire braid. It is taught that these protrusions can be thermally fused to a compatible thermoplastic cover material in a subsequent cover crosshead extrusion process. The disadvantages of this procedure as an optimum means of hose construction are important and are as follows:
Cover material is chosen for maximum flexibility and abrasion and wear resistance. Typical cover materials such as polyurethanes have flexural moduli of 7000 PSI compared to nylon or co-polyester core tube materials with flexural moduli of 30000 to 50000 PSI. Other cover materials are thermoplastic elastomers such as Santoprene, which is a trademark for such material produced by Monsanto Industrial Chemicals Co. The cover is thus much more flexible than the core tube material. Optimally, core tube material is chosen for toughness, chemical resistance and high tensile strength. Typical core tube materials are Hytrel, which is a trademark for a DuPont Co. thermoplastic co-polyester and nylon. With core tube material chosen to be less flexible than cover material, it is furthermore stiffened when it is mechanically oriented by flow through small reinforcement openings. Thus, the construction described by Chermak will tend to stiffen the hose excessively by interlocking high tensile strength and high flexural moduli material between the braided strands and reducing the mobility of the wires accordingly. This will disadvantageously affect the flexibility and service life of the hose, particularly when the hose is subjected to pulsating pressures in service. This effect is increased because of the reduction in average core tube thickness due to the loss of material that expands outward through the reinforcement.
The cover in Chermak is extruded over a wire braid and is supposed to fuse with the core tube protrusions. If the core tube is nylon or co-polyester, however, and the cover is urethane, then the tube and cover materials will not physically wet each other sufficiently to bond unless the cover extrusion is at an impractical high temperature sufficient to melt the core tube surface to achieve mutual fusion. For example, the melt point of Hytrel 5556, a commonly used core tube material is about 412.degree. F., while the typical urethane extrusion temperature is less than 375.degree. F. Clearly, the urethane will not thermally fuse with the Hytrel in this process. Further, even if the melt temperatures were closer together, the economical line speed for cover extrusion is so fast that the tube protrusions could not be heated sufficiently to soften or melt before the cover was quenched in the cooling tank just downstream of the crosshead extrusion die.
Relative to investment economy, it is noted that one cover extrusion line has the capacity to match 5 to 8 braiders in hose throughput. This means that in the case of the Chermak process, 5 to 8 induction heaters, an expensive equipment item, might have to be provided, one at each braider line to match the capacity of one extruder line. The alternative is an extra "off-line" process just to induction heat the wire braid as described by Chermak which would also be a greater operating cost than that required in the present invention.
Another disadvantage of the prior art process has to do with the so-called "hybrid" hose constructions in which there is an inner textile braid and an outer steel wire braid. Such a hose construction is described in U.S. Pat. No. 4,384,595 of Washkewicz et al. It is not desirable in this construction to cause tube protrusion through the inner braid as it would interfere with yarn mobility and prevent stress transfer from the inner to the outer braid. All of these objections are overcome in the present invention as will be made clear hereinafter.